Block storage

Using shared storage for ephemeral storage is one way to achieve a persistence capability for virtual instances, but the more traditional way is to instantiate a Glance image on a Cinder volume. Instances that are backed by Cinder volumes can be live-migrated, live-snapshotted, and evacuated with their state intact. There are Cinder drivers for almost every major storage product on the market these days, and it is common to configure multiple backends within a single OpenStack deployment. This allows the OpenStack tenant to choose an appropriate backend for their workload. For example, a database instance might have its operating system reside on a relatively inexpensive ephemeral disk, but locates the data on a highly performant SAN storage attached by a fiber channel. If the instance fails, the root disk is discarded and a new instance attaches itself to the persistent Cinder volume. Cinder volumes are also attractive as root disks for instances that are being migrated from traditional virtualization platforms. The user experience for Cinder-backed instances is very similar to these platforms.

The selection of storage backends for Cinder implementations is a relatively straightforward process. Cloud architects will want to leverage the relationships and knowledge that an organization's storage architects already have. Storage devices are relatively expensive, and PoCs should be run on existing hardware devices where possible. Cinder drivers for commonplace devices, such as NetApp filers and EMC SANs, have some interesting features, which can be proven out in a lab. Vendors of more exotic hardware devices are typically willing to lend hardware for PoCs as well.

Most of the organizations that we've worked with have tested software-based storage as a part of their OpenStack implementations, and many have gone on to adopt software-based storage for at least one of their storage tiers. Ceph, in particular, has a tight integration with OpenStack for image, ephemeral, and block storage. Ceph has the advantage of providing deduplication when used in this configuration. This, combined with its usage of commodity hardware, makes it an extremely attractive option from a cost perspective.